This description relates to high voltage direct current (HVDC) transmission systems, and, more particularly, to HVDC converter systems and a method of operation thereof.
High voltage direct current (HVDC) electrical power transmission, in contrast with the prevalent alternating current (AC) systems, exhibits benefits of cost and loss reduction in the long-distance transmission of electrical power. Throughout the world, electrical power is traditionally distributed with high voltage AC. High voltage is used for transmission because the loss of energy during transmission is proportional to the amount of electrical current squared (I2R losses). Thus, raising the voltage instead of raising the current allows more energy to be transmitted without significantly increasing the transmission losses. AC was selected for power transmission in the early days of electrification instead of direct current (DC) because it was much easier to transform between different voltage levels with alternating current than with direct current.
Although alternating current is used for most electrical energy transmission, alternating current has its own set of problems. Alternating current generally requires more conductors to carry a similar amount of electrical power than direct current. Alternating current can suffer a ‘skin effect,’ where much of the power transmission is carried by the outer surface of the conductor instead of being uniformly carried by the conductor, thus resulting in increased transmission losses as compared to direct current transmission. Furthermore, it can be very difficult to transmit electrical power with alternating current with undersea or underground cables due to the associated increased cable capacitance. Thus, for many long-distance electrical energy transmission tasks, high-voltage direct current is used instead of alternating current. Direct current transmission is also able to connect asynchronous power grids through a DC hub, which permits a controlled flow of energy while also functionally isolating the independent AC frequencies of each side and thereby preventing fault propagation. For example, the Eastern Interconnection and Western Interconnection are the two major alternating-current (AC) electrical grids in North America. The Texas interconnection is one of several minor interconnections with respect to the Eastern and Western Interconnection. The Eastern, the Western and the Texas Interconnections may be connected via HVDC interconnection links. Geographically overlapping but electrically isolated asynchronous grids can be connected for the same purpose as above using “Back-to-Back” converters configuration requiring no additional transmission lines. Back-to-Back HVDC system can be seen as a specific case of HVDC transmission system.
HVDC transmission lines can also carry electrical energy over long distances with transmission losses significantly less than alternating current transmission losses. For example, high-voltage direct current transmission line losses are typically 30 to 40% lower than alternating current transmission line losses at the same voltage levels. Alternating current transmission lines are limited by their peak voltage levels but do not transmit much power at those peak levels, whereas direct current can transmit full power at the peak voltage level. Furthermore, because direct current does not involve multiple phases nor suffers from the skin effect, direct current transmission lines can have fewer conductor lines and smaller conductor lines. As a result dimension and costs of the transmission towers is reduced along with issues associated with the right-of-way. Additionally, reactive power issues that affect alternating current transmission, do not affect direct current transmission.
However, HVDC transmission is generally avoided unless the power is being transmitted by an undersea cable or over a very long distance. High-voltage direct current is generally avoided because the conversion equipment is very complex and expensive. Thus, even though direct current provides significant efficiency advantages for electrical energy transmission, direct current is rarely used for electricity transmission. At least some known HVDC transmission systems include conventional conversion equipment that typically includes a multi-phase AC-to-DC converter, a long distance DC power conductor, such as, but not limited to an electrical cable for transmission of the electrical power, and a multi-phase DC-to-AC inverter on the load end of the system. Switching valves in the multi-phase AC-to-DC converter and multi-phase DC-to-AC inverter are typically silicon-based and subject to relatively low voltage and current ratings. To increase the ratings of the system to a level useful for power transmission systems, many such valves are coupled in electrical series and/or electrical parallel. Although such connections increase the ratings of the multi-phase AC-to-DC converter and the multi-phase DC-to-AC inverter, such connections also increase the complexity of commutation of the valves and the space requirements of the components that make up the conversion system.
The state-of-the-art of HVDC converter technology has evolved from the line commuted converter (LCC) HVDC using thyristors, to voltage source converter (VSC) HVDC technology, and most recently to the modular multilevel converter (MMC) HVDC technology and their hybrid combinations. The LCC HVDC is a current source system, in which the thyristor is used. Because the thyristor cannot be forced to turn off, this kind of system relies on the grid for commutation. Therefore, it consumes a large amount of reactive power compensation as well as requires a strong grid environment. In addition, thyristors have a relatively low blocking voltage capability. For example, HVDC thyristor power semiconductors are rated for approximately eight kV.